This invention relates to a control system for a parking brake which inhibits the actuation of a solenoid valve caused by intermittent changes in current supplied to the solenoid valve resulting from the operation of a vehicle to prevent the release of pressurized fluid which holds a parking brake in a released position.
Parking brake systems having a spring apply and a hydraulic or air release are known as illustrated by the structure disclosed in the following U.S. Pat. Nos.: 3,802,745; 3,827,758; 4,861,115; 5,310,251; 5,370,449. In such parking brake systems, a solenoid valve is usually connected in series with a parking brake control and a transmission control console. The parking brake system is activated by pushing or pulling a plunger control which is normally located on a dash of the vehicle. When the selector for transmission of the vehicle is moved to a xe2x80x9cparkxe2x80x9d position, the transmission is actually in a neutral condition while the parking brake may be in either the applied or un-applied position. The parking brake is applied by a spring which acts through a linkage to provide a mechanical force that moves friction linings into engagement with a drum or disc brake to effect a parking brake application. In order to release the parking brake, a solenoid valve is actuated to allow pressurized fluid to be communicated to an actuator which acts on a diaphragm or piston associated with the linkage. The hydraulic force developed by the pressurized fluid acts on piston to compresses the spring and thereafter move the friction linings away from the drum or disc brake. With the spring un-applied, the wheels of the vehicle can now roll and the vehicle driven.
A problem for such a system is that an unexpected loss of electrical power in a general electrical system may create a hazardous situation through an inadvertent actuation of the parking brake. This problem was partially recognized in U.S. Pat. No. 5,370,449 where a parking brake control system was disclosed which prevents the actuation of the parking brake during certain conditions. In this system an electronic control unit provides a input to momentary energize a first solenoid to a desired position where pressurized fluid is released from an actuator to allow a spring to mechanically move a brake to a brake apply position. The first solenoid is maintained in the brake apply position by a mechanical detent after de-energization by the electronic control unit. The electronic control unit receives inputs, such as engine speed, ground speed, transmission gear selection and parking gear selection, relating to the operation of the vehicle and only if certain conditions are present supplies a second solenoid with an input to allow pressurized fluid to be supplied to an actuator which acts on the spring and allow the parking brake to be released. While this structure does prevent the release of a parking brake of a vehicle from a parked position, it does not address actuation of a parking brake through a loss of power once the vehicle has been in motion.
It is an object of the present invention to provide a control system for a parking brake with a redundantly actuated solenoid valve whereby the release of the parking brake from an actuation position is achieved by movement of a plunger through a combined magnetic force developed by first and second coils of the solenoid valve to allow pressurized fluid to overcome a spring while a magnetic force developed in either the first and second coils is sufficient to maintain the plunger in the released position.
In more particular detail, the control system for a parking brake includes an actuator assembly which is responsive to a resilient member for moving the parking brake into a brake apply position and to a hydraulic arrangement for moving the parking brake into a brake release position. The hydraulic arrangement including a housing with a bore therein for retaining a plunger of a solenoid valve. The bore has an inlet port connected to a source of pressurized fluid, an outlet port connected to the actuator assembly and a reservoir or relief port connected to a reservoir. The plunger has a first position in the bore whereby the outlet port is connected with the relief port to permit communication of pressurized fluid from the actuator assembly when it is desired that the parking brake be in a brake apply position. The plunger has a second position in the bore whereby the inlet port is connected to the outlet port to communicate pressurized fluid from a supply source to the actuator assembly. The pressurized fluid, after overcome the resilient member, moves the parking brake to a release position. A coil of the solenoid valve is connected to receiving electrical current from an electrical source (battery) on activation of a parking brake switch. The flow of electrical current in the coil develops a magnetic force which acts on and moves the plunger from the first position to the second position. The solenoid valve is characterized by the coil having a first circuit which directly receives a first current from the electrical source in response to the actuation of the parking brake switch and a second circuit which receives a second current from the electrical source by way of an ignition switch in response to the actuation of the parking brake switch. The second current can be effected by variation caused by changes in the operational conditions of the vehicle and as a result a first magnetic force developed in first circuit may differ from a second magnetic force developed in the second circuit of the coil. However, the first magnetic force and the second magnetic force are combined and jointly move the plunger from the first position to the second position. Thereafter, either the first or second magnetic force is sufficient to independently hold and maintain the plunger in the second position. Thus, after a vehicle is moving and even if either the first or second magnetic forces do not develop in the coil, a remaining magnetic force is sufficient to hold the plunger from moving to the first position which would allow pressurized fluid to be released from the actuator and allow the resilient member to apply the parking brake.
An advantage of this control system resides in a independent and redundant actuation of first and second circuits to develop first and second magnetic forces in a coil of a solenoid valve to move a plunger which controls the communication of pressurization to an actuator for releasing a spring applied parking brake with either the first or second magnetic force being sufficient to prevent an inadvertent actuation of the parking brake.
An object of this invention is to provide a control system with first and second electrical circuits wherein electrical current is communicated to a coil in a solenoid valve for developing first and second magnetic forces which jointly act on and move a plunger to allow pressurized fluid which acts on and moves a resilient member to release a parking brake and either of the first and second magnetic forces is sufficient to maintain and hold the plunger in this position to assure that the parking brake remains in the released position.
A further object of this invention is to provide a parking brake control system with a solenoid valve activated by combined first and second magnetic forces developed in a coil which move a plunger to a applied position to allow pressurized fluid to be presented to a parking brake to overcome a spring force and release the parking brake from an applied position, with either of the first or second magnetic force being sufficient to maintained the plunger in the applied position should a failure occur in a development of the other magnetic force which would allow the plunger to move to the applied position until the vehicle is brought to a stop where a spring applied force effects a parking brake application but being insufficient to move the plunger once the vehicle is stopped and thereafter the release the spring applied force until the failure is corrected.